In order to provide a next-generation long-distance large-capacity communication system, research and development have been made on a transmitter that generates modulated optical signals using digital signal processing. For example, the digital signal processing generates signals for a multi-level modulation format such as QPSK and QAM. Also, for example, the digital signal processing can generate signals for suppressing dispersion of an optical transmission line (that is, signals for dispersion pre-equalization). Then, an optical modulator is driven by the signals generated by the digital signal processing to generate large-capacity multi-level modulated optical signals.
FIG. 1 illustrates an example of an optical transmitter. An optical transmitter 1 illustrated in FIG. 1 includes a digital signal processor 11, D/A converters 12a and 12b, drivers 13a and 13b, a light source 14, and an optical modulator 15. The digital signal processor 11 generates a drive signal I and a drive signal Q from input data. The D/A converters 12a and 12b covert the drive signal I and the drive signal Q to an analog signal respectively. The drivers 13a and 13b drive the optical modulator 15 with the drive signal I and the drive signal Q outputted from the D/A converters 12a and 12b respectively. The light source 14 generates continuous wave light.
The optical modulator 15 includes an I arm modulation unit and a Q arm modulation unit. The continuous wave light generated by the light source 14 is split by an optical splitter and guided to the I arm modulation unit and the Q arm modulation unit. The I arm modulation unit modulates the continuous wave light in response to the drive signal I to generate an optical signal. The Q arm modulation unit modulates the continuous wave light in response to the drive signal Q to generate an optical signal. The two optical signals are combined to generate a modulated optical signal.
The optical modulator 15 is designed so that the phase difference between the I arm and the Q arm is π/2+nπ (where n is any integer including zero). More specifically, a bias voltage supplied to a phase shifter 15a is controlled so that the phase difference between light propagating through the I arm and light propagating through the Q arm is π/2+nπ. Note that the method of controlling the phase difference between the I arm and the Q arm of the optical modulator so as to be π/2 is described, for example, in Japanese Laid-Open Patent Publication No. 2007-82094, Japanese Laid-Open Patent Publication No. 2009-246578, and Japanese Laid-Open Patent Publication No. 2007-259426.
However, in the adjustment of the phase difference between the I arm and the Q arm of the optical modulator (hereinafter referred to as an “I-Q phase difference” or a “phase shift amount of the phase shifter”), π/2+nπ and 3π/2+2nπ are not distinguished. For example, the optical transmitter may generate the modulated optical signal in a state in which the I-Q phase difference is controlled to be either π/2+2nπ or 3π/2+2nπ. In light of this, the optical receiver decides whether the I-Q phase difference is either π/2+2nπ or 3π/2+2nπ, and then recovers data from the modulated optical signal. The optical receiver for receiving the modulated optical signal is described, for example, in Japanese Laid-Open Patent Publication No. 2006-270909.
The next-generation optical transmission system can provide an optical signal with various characteristics using the digital signal processing as described above. For example, the digital signal processing for generating a drive signal from the input data can perform dispersion pre-equalization, frequency offset addition, or the like.
However, when a parameter assuming that the I-Q phase difference is π/2+2nπ is supplied while the I-Q phase difference is controlled to be 3π/2+2nπ, the transmission characteristics of the optical signal may be degraded. For example, when chromatic dispersion assuming that the I-Q phase difference is π/2+2nπ is supplied while the I-Q phase difference is controlled to be 3π/2+2nπ, cumulative chromatic dispersion to be detected by the optical receiver may be greater than a case where wavelength pre-equalization is not performed.